Small broadband antennas conformable to curved platforms have become increasingly more important for both military and commercial applications. The broadband requirement is driven by the proliferation of wireless systems and the need for high speed. The smallness of an antenna is measured by its operating free-space wavelength; generally, an antenna is electrically small if its largest dimension is less than ½ free-space wavelengths, especially if a broad bandwidth, say, over 20%, is required. The conformability feature, defined as having minimal protrusion and intrusion to the surface of the platform on which the antenna is mounted, is desirable and even necessary, especially for airborne platforms.
Now, broadband and smallness/conformability are inherently conflicting requirements for antennas. The bandwidth of an antenna is limited by its size, shape, and the interference of proximate objects. Although the class of frequency-independent (FI) antenna had been invented from late 1950s through 1960s, and was well documented in the literature (e.g., DuHamel and Scherer, 1993; Mayes, 1988), these antennas were designed with no reference to their conformability nor their mounting platform, both of which restrict the size and shape, as well as the radiation property, of the antenna. Note that an antenna is necessarily connected with a feed cable and a transceiver, which is a de facto platform that cannot be ignored, especially if the platform (and consequently the antenna) is electrically small.
Around 1970, a conformal antenna called the microstrip patch antenna was invented, which has a ground plane as part of its design and is thus amenable to mounting on a platform with a conducting or nonconducting surface. Unfortunately the microstrip patch antenna is a narrowband antenna. It took another two decades before a broadband version was invented. It was the spiral-mode microstrip (SMM) antenna (Wang and Tripp, 1991; Wang and Tripp, 1994). Since 1990, significant progress has been made in the SMM antenna (Wang, 2000; Wang et al, 2006); and additional techniques using planar FI antennas, notably the miniaturized slow-wave (SW) antenna (Wang and Tillery, 2000), have been developed. In addition to an octaval bandwidth of up to 10:1 or more, the multiplicity of radiation features in these antennas provide the unique capability of multifunction, such as dual-polarization, rarely available in other antennas.
A common feature of these patented designs, from the microstrip patch antenna to SMM antenna to SW antenna, is the inclusion of a fairly planar ground plane placed very close to, and parallel to, a fairly planar surface radiator. The inclusion of a conducting ground plane in these antennas makes them amenable to conformal mounting on the surface of a platform such as an airplane or a ground vehicle. However, for a platform that is irregularly shaped, and/or has a small size and a small radius of curvature (in terms of the operating wavelength), these antennas have thus far been unable to satisfy most conformability requirements.
Additionally, the gain bandwidth of an antenna is fundamentally limited by its electrical size (namely, size in wavelength); thus broadband is difficult to achieve when the antenna is electrically small. This theory on antenna gain-bandwidth limitation due to the antenna size was developed by Chu six decades ago (1948). Since then, many prominent scholars in electromagnetic theory have visited and revisited this problem, and all with confirming findings. Today, the Chu equation for the gain-bandwidth limitation of an antenna of a given size remains essentially intact.
Recently, this inventor noted some major shortcomings and ambiguities in the Chu theory when applied to real-world problems (Wang, August 2005; Wang, March 2006). These severe shortcomings of the Chu theory are rooted in its basic assumptions which are overly narrow and incompatible with most real-world problems. First, an antenna is rarely an object isolated in space; its specific size becomes ambiguous when it is mounted on a platform. Since an antenna is always connected to a transmission line feeding a transceiver, its extent and size become ambiguous, especially if it is electrically small. In fact, in some designs of electrically small antennas the main radiator is the platform or transceiver, not the antenna per se.
Second, in the Chu theory the antenna problem was formulated restrictively (strictly speaking, inadequately) as an antenna with an external matching network, with single-port connections between them and the transceiver. The employment of a matching structure in the antenna aperture or the use of multiple ports would present a problem not subject to the Chu limitation.
Third, the Chu theory is applicable only to high-Q (quality factor) narrowband antennas because it is based on the inverse relationship between Q and bandwidth, which rapidly becomes invalid as Q decreases below about 4. Thus, the Chu theory breaks down for broadband (low Q) antennas which are typically of the non-resonant type.
Fourth, the unrealistic assumption of zero dissipative loss makes it unamenable to the design approach which optimizes gain-bandwidth at a small sacrifice of dissipative loss.
This inventor reported in the two papers cited earlier that conformal traveling-wave (TW) antennas, such as the SMM antenna and the SW antenna, are not subject to the overly restrictive Chu limitation. For these conformal TW antennas, octaval bandwidth (defined as the ratio of the upper bound and lower bound of the operating bandwidth) over 10:1, and exceeding the Chu limitation, is feasible. The practical bandwidth limitation on the upper frequency bound is largely due to its radiation property (pattern and polarization); and at its lower frequency bound is due to its impedance.
However, these conformal TW antennas exceeding the Chu limitation are confined to the SMM antenna and the SW antenna, both of which have a conducting ground plane and a radiator fairly planar and spaced a constant distance apart. Recently, this inventor conceived the present invention, which potentially has superior performance and/or form factor over prior-art approaches.
Additionally, the present invention is an innovation which achieves broadband and conformability for a given platform of small size and curved surface, and also reduces the size of the antenna by coupling the traveling wave to the surface of the platform to effect radiation at the lower end of the operating frequencies.